Theresa May is to chair an emergency Cabinet meeting to discuss the latest crisis in Syria amid signs she is preparing to join US-led air strikes against the regime of president Bashar Assad.
The prime minister summoned ministers to No.10 after saying "all the indications" were that the regime was responsible for an alleged chemical attack on its own people last weekend.
The Ministry of Defence refused to comment on a report in The Daily Telegraph that Royal Navy submarines had been ordered into range to launch Tomahawk cruise missile strikes as early as Thursday night.
"We don't comment on submarine movements," a spokesman said.
Separately, Downing Street would not be drawn on claims Mrs May was preparing to authorise UK forces to strike against Assad without first seeking a vote in Parliament.
But Conservative MP Sarah Wollaston, talking Julia Hartley-Brewer, said: "The prime minister doesn’t have to call a vote in Parliament.
"It will depend on how much she is able to say publicly. There may be briefings that she’s been given that she’s not at liberty to make public.
"Unless you’re voting knowing absolutely all the facts and background, that’s not a valid vote."
Mrs May has faced calls to wait for unequivocal proof of a chemical attack by the Assad regime before committing British forces to retaliatory action.
Labour leader Jeremy Corbyn joined other opposition parties - as well as some Conservative backbenchers - in insisting MPs must be have a say on any British involvement in military action.
But Mrs May faces growing impatience from Washington, where President Donald Trump tweeted the missiles "will be coming".
Ms Wollaston argued that should the prime minister decide to act without a Parliamentary vote it is not because of a weakened position at Westminster and not having a majority.
"If anything, I think a lot of people will be looking at this situation and thinking thank goodness we’ve got Theresa May in charge and not Jeremy Corbyn."
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